The quinacridone family of compounds and their pigmentary properties are well known. The pigmentary quinacridones include the parent compound, quinacridone, and various disubstituted quinacridones, including 2,9-dichloroquinacridone.
It is well known in the pigments art that substituted quinacridone pigments can be prepared by the oxidation of the corresponding dihydroquinacridones. The product of such an oxidation, known as a crude quinacridone, is generally unsuitable for use as a pigment and must be further processed to develop the requisite pigmentary properties, such as particle size, particle shape, polymorphic phase and tinctorial strength.
The crude quinacridone is commonly convened to a pigmentary form by milling the crude quinacridone with large quantities of inorganic salt followed by extraction of the resulting mill powder, or by dissolving the pigment in large quantities of concentrated sulfuric acid and drowning the solution into water (acid pasting). Since these multistep procedures generally require a diversity of operations conducted at elevated temperatures in acidic environments, simpler, more economical procedures for preparing pigmentary quinacridones are highly desirable.
It is known from U.S. Pat. No. 4,197,404 that o-carboxybenzamidomethylquinacridone, which is a hydrolysis product of 2-phthalimidomethylquinacridone, can be utilized as a particle growth inhibitor to prepare higher strength .gamma.-quinacridone directly from the oxidation of .beta.-dihydroquinacridone. However, the reference discloses that when more than 1.0% of o-carboxybenzamidomethylquinacridone was used, the oxidation of the .beta.-dihydroquinacridone was inhibited. This results in incomplete conversion of D-dihydroquinacridone to .gamma.-quinacridone.
The primary object of this invention is to prepare pigmentary 2,9-dichloroquinacridone directly during synthesis, without the need for aftertreatments. This objective is achieved by the discovery that the inhibition of the oxidation of dihydroquinacridone, which results from adding more than 1 percent of the particle growth inhibitor, does not occur during the oxidation of 2,9-dichloro-6,13-dihydroquinacridone. Therefore, additional amounts of particle growth inhibitor can be added to the reaction mixture in order to obtain pigmentary 2,9-dichloroquinacridone directly from the synthesis.